The present invention relates in general to a method and apparatus for determining the location of a short circuit or an open circuit in an electrical cable, based on impedance measurements made at one end of the cable.
The Space Shuttle uses dedicated signal conditioners (DSCs) for the purpose of conditioning transducer outputs and other signals to make them compatible with orbiter telemetry, displays and data processing systems. The DSCs are located throughout the orbiter, often in difficult to access locations in the vehicle fuselage. When troubleshooting a potential instrumentation problem, personnel frequently have to demate cables to verify that the cables are not the source of the problem. However, once a cable is demated, all DSCs and other systems which have a wire passing through the cable's connector have to be retested after the cable is reconnected. Due to their inaccessibility, the DSCs sometimes have to be removed in order to check them. This results in many man hours of revalidation testing on systems that were unrelated to the original problem. The cost to the Shuttle program for these retesting procedures is exorbitant. A system which allows cable continuity to be checked non-intrusively without demating the cables would therefore save many hours of testing and substantially reduce testing costs.
One known technique for non-intrusively checking cable continuity is time domain reflectometry (TDR). In TDR, a high frequency pulse is injected into one end of the cable. A discontinuity in the cable will cause a reflection of the pulse that can be detected back at the end where the pulse was injected. The location of this discontinuity can be determined by measuring the time interval between when the pulse is injected into the cable, and when the reflected pulse is detected. Also, the polarity of the reflected pulse is indicative of whether the discontinuity is a short circuit or an open circuit.
Unfortunately, TDR is not feasible for detecting cable faults in the Space Shuttle. This is because the faults are typically only a few meters or less from the injection point, and TDR is ineffective at these distances due to the extremely short total travel time (e.g., a few nanoseconds) of the high frequency pulse.
In view of the foregoing, a need therefore exists for a non-intrusive cable testing technique which provides the accuracy necessary for use in the Space Shuttle environment.